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Subject: Re: [PATCH] mm/madvise: allow MADV_DONTNEED to free memory that is
 MLOCK_ONFAULT
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Vlastimil Babka <vbabka@suse.cz>, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Mel Gorman <mgorman@suse.de>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        linux-api@vger.kernel.org, emunson@mgebm.net
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From:   Jason Baron <jbaron@akamai.com>
Message-ID: <4ab6d77a-3032-3ffb-d556-b736f6b983e6@akamai.com>
Date:   Thu, 28 Jun 2018 16:20:54 -0400
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On 06/20/2018 07:00 AM, Michal Hocko wrote:
> On Fri 15-06-18 15:36:07, Jason Baron wrote:
>>
>>
>> On 06/13/2018 03:15 AM, Michal Hocko wrote:
>>> On Wed 13-06-18 08:32:19, Vlastimil Babka wrote:
> [...]
>>>> BTW I didn't get why we should allow this for MADV_DONTNEED but not
>>>> MADV_FREE. Can you expand on that?
>>>
>>> Well, I wanted to bring this up as well. I guess this would require some
>>> more hacks to handle the reclaim path correctly because we do rely on
>>> VM_LOCK at many places for the lazy mlock pages culling.
>>>
>>
>> The point of not allowing MADV_FREE on mlock'd pages for me was that
>> with mlock and even MLOCK_ON_FAULT, one can always can always determine
>> if a page is present or not (and thus avoid the major fault). Allowing
>> MADV_FREE on lock'd pages breaks that assumption.
> 
> But once you have called MADV_FREE you cannot assume anything about the
> content until you touch the memory again. So you can safely assume a
> major fault for the worst case. Btw. why knowing whether you major fault
> is important in the first place? What is an application going to do
> about that information?
> 

Fair enough, I think that means you end up with a MADV_FREE_FORCE to
support that case? As I said I worked around this by using tmpfs and
fallocate(FALLOC_FL_PUNCH_HOLE). However, I still think there is a
use-case for doing this for anonymous memory, to avoid the unlock() calls.

The use-case I had in mind was simply an application that has a fast
path for when it knows that the requested item is locked in memory.

Thanks,

-Jason